A police inquiry into the cause of the M5 crash in which seven people died and another 51 were injured has concluded drifting smoke from a nearby fireworks party was not to blame.

Emphasis has instead shifted to consider driver error the probable trigger, possibly a vehicle braking heavily as it encountered a dense fog bank just ahead of the main body of the accident.

However, no charges are expected to be brought. A final decision will made by Crown Prosecution Service lawyers in the next few months.

The four-month investigation has exonerated Taunton Rugby Club and Firestorm Pyrotechnics, which mounted a display alongside the M5.

In the aftermath of the crash in wet, foggy conditions last November, Avon and Somerset police assistant chief constable Anthony Bangham was quick to claim the inquiry was “criminal” and pointed the finger of suspicion at the club and Geoff Counsell, the fireworks company boss, whose event finished minutes before the accident on November 4.

Lawyers representing the club and Mr Counsell considered the comments prejudicial and are still considering legal action and a professional complaint against Mr Bangham.

Both were subjected to a hate campaign which has taken a “severe emotional and mental toll” on rugby club officials and Mr Counsell.

Traffic officers, who were among the first on the scene at around 8.30pm, were also taken aback by the claim as they originally considered fog and speed the main factors.

Last December it was revealed the weather conditions were such that any smoke from the firework display would have been carried away from the site of the crash.

Now, a senior police source has indicated rugby club officials and Mr Counsell were considered “witnesses not suspects” and no criminal charges will be brought.

The source added: “It has been an incredibly lengthy, complex and demanding inquiry. Ultimately it has been so rigorous and so detailed it is almost impossible to be categoric about exactly what happened.”

Instead, the main thrust of the police inquiry has looked at the possibility that an error by a driver, possibly one who had joined the motorway at junction 25, was the catalyst.

An accident investigator said: “It is well known among truckers as a difficult slip road to negotiate. It is uphill which means lorries are travelling very slowly when they join.”